1. Field of the Invention
This invention relates to digital data transmission, and in particular it relates to digital data decoders. The following description is based on the GSM cellular communications system for which the invention is of particular utility. It will be apparent to those skilled in the art, however that the invention may be applied to other systems of digital data transmission.
2. Description of the Related Art
Reference is made to U.S. Pat. Nos. 5,598,506 and 5,596,678 to Wigren et al U.S. Pat. No. 5,557,639 to Heikkila and xe2x80x9cMobile Radio Communicationsxe2x80x9d published by John Wiley and Sons, Raymond Steele (Ed.) for a description of the prior art and technological background. The following abbreviations are used herein:
GSMxe2x80x94Global System for Mobile communications, (formerly Groupie Special Mobile)
BCCHxe2x80x94Broadcast control channel
TCH/Fxe2x80x94Traffic channel full rate
CRCxe2x80x94Cyclic redundancy check.
BFIxe2x80x94Bad frame indication
MSxe2x80x94Mobile station
PBERxe2x80x94Pseudo bit error rate
The GSM cellular communications system uses the Full Rate speech codec as default. The full rate speech codec encodes l3 kHz samples into 260 bits containing 76 parameters. These 260 bits are divided into two groups based on their subjective importance to speech quality.
The 78 least important bits are known as class II bits and are unprotected. Corruption of these class II bits has little audible effect on speech quality. The most important 182 bits are known as class I bits protected by a half rate convolutional code. The class I bits are further subdivided into Ia and Ib, such that the most significant 50 bits (Ia) are additionally protected by a 3 bit cyclic redundancy check (CRC).
In order to prevent unpleasant audio artifacts during speech transmission, any frame erasure mechanism must detect all frames with class la errors and frames with more than a certain number of class lb errors, as precisely and efficiently as possible for all propagation channel types.
Network operators wish to maximize the capacity and quality of their networks. One of the ways in which this can be achieved is to employ slow frequency hopping. A slow frequency hopping channel follows a cyclic pseudo-random hopping sequence, each burst being transmitted on a frequency which is different from the frequency of the previous burst. A performance gain is achieved from the frequency diversity of the hopping sequence.
The use of slow frequency hopping also allows greater re-use of the frequencies allocated to an network operator, thereby increasing the capacity available. One side effect of slow hopping and frequency re-use is the creation of a special class of propagation channels, known as Telstra channels, so called because the Australian network operator Telstra implemented its own mobile station (MS) performance test for this type of channel.
This class of propagation channel (Telstra) is characterized by a slow frequency hopping channel, hopping across n (typically n=4) frequencies, one of which has a high level of co-channel interference present (typically xe2x88x9210 to xe2x88x9220 dB) which may arise from an adjacent cell""s BCCH broadcast or traffic channels. The effect of this interference is that for the burst affected by it, the probability of the bits being in error tends toward 50% (i.e. essentially random) and the other nxe2x88x921 frequencies have a low probability of error.
Because of the interleaving and re-ordering of the TCH/FS channel, these erroneous bits are evenly distributed across the whole speech frame, interlaced with the correct bits from the other bursts. This is in direct contrast to ordinary (non-Telstra like) channels where all bursts contributing to the speech frame are equally likely to contain errors so that after the interleaving has been removed and re-ordering performed, it is likely that non-evenly distributed bits in the encoded speech frame will be in error. The r=xc2xd, K=5 code used in this GSM coding scheme is powerful enough successfully to correct errors on a Telstra channel. This behavior was originally noted by Telstra for a 1:4 hopping channel.
The conventional frame erasure algorithm used for the GSM TCH/FS channel is composed of two individual tests: a 3 bit CRC check and a pseudo bit error rate (PBER) threshold. The PBER is calculated by re-encoding the decoded class I bits and comparing them, bit by bit, with the original received bits. The CRC check is computed over the class la bits of the speech frame. The number of estimated errors is calculated for all of the encoded class I bits.
The frame erasure algorithm proceeds as follows:
1. Convolutionally decode the 189 encoded class I symbols to give the 182 class I bits +3 CRC check bits +4 tail bits.
2. Perform a CRC check on the class Ia bit. If the CRC check fails, mark the frame for erasure.
3. Re-encode the 182 class I bits +3 CRC check bits +4 tail bits and then perform a bit by bit comparison between the re-encoded symbols and the original received symbols, to compute the number of differences.
4. If the number of differences exceeds the PBER threshold then the frame is marked for erasure.
This algorithm works well for xe2x80x9cnon-Telstra likexe2x80x9d channels. However, the performance of the CRC check is dependent on the PBER threshold value. The lower the PBER threshold, the more reliable the CRC check becomes. A PBER threshold value typically of between 45-60 (bit/frame) is required.
This algorithm does not work well for xe2x80x9cTelstra likexe2x80x9d channels because they require a PBER threshold which is too high for the CRC check to be reliable for use with non-Telstra channels. This is because the convolutional code can correct a larger number of errors for a Telstra channel due to the even, periodic distribution of errors and if the PBER threshold is set too low for xe2x80x9cTelstra likexe2x80x9d channels, too many error free speech frames will be erased.
Objects of the present invention includes better frame erasure performance for Telstra 1:3 and 1:4 channels and better error checking of class Ib bits for all channels.
According to the invention there is provided a method of identifying a frame for erasure in a digital data transmission system comprising, after de-interleaving and equalization, the setting of a PBER threshold in accordance with the result of a comparison of the convolutionally decoded received symbols with a copy of the convolutionally decoded received symbols, said copy having been bit reversed, convolutionally decoded and further bit reversed, and comparison of the received symbols with the re-encoded convolutionally decoded received symbols to provide an estimated number of errors in the received symbols and where said estimated number of errors exceeds the PBER threshold setting, the received frame being marked for erasure.